Many traditional imager readers, such as hand held and fixed mounted bar code and machine code readers, employ charge-coupled device (CCDs) based image sensors. A CCD based image sensor contains an array of electrically coupled light sensitive photodiodes that convert incident light energy into packets of electric charge. In operation, the charge packets are shifted out of the CCD imager sensor for subsequent processing.
Some image readers employ CMOS based image sensors as an alternative imaging technology. As with CCDs, CMOS based image sensors contain arrays of light sensitive photodiodes that convert incident light energy into electric charge. Unlike CCDs, however, CMOS based image sensors allow each pixel in a two-dimensional array to be directly addressed. One advantage of this is that sub-regions of a full frame of image data can be independently accessed. Another advantage of CMOS based image sensors is that in general they have lower costs per pixel. This is primarily due to the fact that CMOS image sensors are made with standard CMOS processes in high volume wafer fabrication facilities that produce common integrated circuits such as microprocessors and the like. In addition to lower cost, the common fabrication process means that a CMOS pixel array can be integrated on a single circuit with other standard electronic devices such as clock drivers, digital logic, analog/digital converters and the like. This in turn has the further advantage of reducing space requirements and lowering power usage.
CMOS based image readers have traditionally employed rolling shutters to expose pixels in the sensor array. In a rolling shutter architecture, rows of pixels are activated and read out in sequence. The exposure or integration time for a pixel is the time between a pixel being reset and its value being read out. This concept is presented in FIG. 2A. In FIG. 2A, the exposure for each of the rows “a” though “n” is diagrammatically represented by the bars 4a . . . 4n (generally 4). The horizontal extent 8 of each bar is intended to correspond to the exposure period for a particular row. The horizontal displacement of each bar 4 is suggestive of the shifting time period during which each row of pixels is exposed. As can be seen in FIG. 2A, the exposure period for sequential rows overlap. This is shown in more detail with respect to the timing diagrams for a rolling shutter architecture shown in FIG. 2B. The second 12 and third 16 lines of the timing diagram represent the reset timing signal and the read out timing signal, respectively, for row “a.” The fourth 20 and fifth 24 lines represent the reset and the read out timing signals, respectively for row “b.” As shown in both FIGS. 2A and 2B, the exposure for row “b” is initiated before the values for row “a” are read out. The exposure periods for adjacent rows of pixels typically overlap substantially as several hundred rows of pixels must be exposed and read during the capture of a frame of data. As shown by the illumination timing signal on the first line 28, the rolling shutter architecture with its overlapping exposure periods requires that the illumination source remain on during substantially all of the time required to capture a frame of data so that illumination is provided for all of the rows.
In operation, the rolling shutter architecture suffers from at least two disadvantages: image distortion and image blur. Image distortion is an artifact of the different times at which each row of pixels is exposed. The effect of image distortion is most pronounced when fast moving objects are visually recorded. The effect is demonstrated in the image shown in FIG. 3 that shows a representation of an image taken with a rolling shutter of a bus image pixels 50 passing through the field of view from right to left. As the top row of bus image pixels 54 of the bus was taken earlier than the bottom row of pixels 58, and as the bus was traveling to the left, the bottom row of bus image pixels 58 is displaced to the left relative to the top row of bus image pixels 54.
Image blur is an artifact of the long exposure periods typically required in a rolling shutter architecture in an image reader. As indicated above, in a rolling shutter architecture the illumination source must remain on during substantially all of the time required to capture a frame of data. Due to battery and/or illumination source limitations, the light provided during the capture of an entire frame of data is usually not adequate for short exposure times. Without a short exposure time, blur inducing effects become pronounced. Common examples of blur inducing effects include the displacement of an image sensor due to, for example, hand shake with a hand held image reader.
What is needed is an image reader that overcomes the drawbacks of current CMOS image readers including image distortion and image blur.